1. Field of the Invention
The present invention relates to a hydraulic pressure control device for an automatic transmission for a vehicle such as an automobile, and more particularly, relates to a hydraulic pressure control device having a reverse stage engagement prevention system for preventing the automatic transmission being set to the reverse stage under a certain vehicle operating condition such as forward traveling at or above a certain speed.
2. Description of the Prior Art
In the art of transmission for a vehicle such as an automobile it has already been proposed to incorporate in the hydraulic pressure control device of the automatic transmission a reverse stage prevention system for the purpose of failsafe such as described above, so that, even if the driver mistakenly carries out a manual shift change operation toward the reverse stage while the vehicle is traveling forward, the automatic transmission is not shifted to the reverse stage.
Such an art is disclosed in Japanese Patent Laying-open Publication Sho 60-37446 (1985). In this prior art, the reverse stage prevention system has a reverse stage disabling valve of a hydraulically operated type for carrying out a switching over operation according to supply or draining of a signal hydraulic pressure thereto, so as selectively to prevent supply of hydraulic pressure to a friction engaging device for setting up the reverse stage and to connect said friction engaging device to a drain. In such a prior art the supply or draining of the signal hydraulic pressure to said reverse stage disabling valve is controlled by a particular solenoid valve. Therefore, the hydraulic pressure control device having such a reverse stage prevention system requires an additional and exclusive solenoid valve for the reverse stage disabling valve, as compared with the transmission in which such a reverse stage prevention system is not provided. Such an addition of an exclusive solenoid valve not only increases the cost of the transmission but also increases the size and weight of the transmission control system, and reduces the operating reliability by increasing the number of items which require preventative maintenance.